Institution: | a Department of Mathematics, Cergy-Pontoise University, 95000 Cergy-Pontoise, France b Department of Mathematics, Lyon 1 University, France c University of Ottawa, 585 King Edward, Ottawa, ON K1N 6N5, Canada d Institut de Mathématiques de Toulouse, UMR CNRS 5219, Université Toulouse 3, 118 route de Narbonne, 31062 Toulouse Cedex 9, France |